1. Field of the Invention
The present invention relates to a device and method for driving optical writing heads which optically write image information on photoconductors by illuminating those photoconductors with the light emitted from light sources in an electrophotographic recording apparatus for performing printing by electrophotography.
2. Description of the Related Art
A color electrophotographic recording apparatus, which creates a color image by forming images of individual color components one after another and superimposing those images on one another, has become known in recent years. The color electrophotographic recording apparatus is a tandem-type one including four image forming units, each of which forms an image in a corresponding one of four colors, i.e., yellow (Y), magenta (M), cyan (C) and black (BK). The color electrophotographic recording apparatus forms a color image by superimposing the images formed by the image forming units on one another. The image forming units illuminate photosensitive drums with the light emitted from light sources, thereby optically writing image information so that electrostatic latent images are formed on the photosensitive drums. The image forming units adhere color toners to the electrostatic images and transfer the toner images thus formed from the photosensitive drums to a sheet of recording paper. The image forming units cause optical writing heads, employing LEDs, semiconductor lasers or the like as the light sources, to illuminate the photosensitive drums with the LED light or the like in order to optically write the image information.
Each of the optical writing heads includes, for example, LEDs which are arranged in a line along a main scanning direction or the direction of a print width, and optically writes the image information on the photosensitive drums. The optical writing performed by the optical writing heads will now be explained as an example, with reference to FIGS. 22 and 23.
FIG. 22 is a diagram exemplifying one of conventional driving circuits. Each of the driving circuits causes the LED array included in a corresponding one of the optical writing heads to emit light. FIG. 23 is a timing chart showing the timing of the operation of the conventional driving circuit illustrated in FIG. 22.
The driving circuit illustrated in FIG. 22 includes a shift register 100, a latch circuit 101, an AND gate 102, a buffer 103 and an LED array 104.
The shift register 100 sequentially acquires bit map data (DATA) from an non-illustrated interface circuit, in synchronization with a clock signal (CLK). The shift register 100 outputs the acquired bit map data to the latch circuit 100 in a parallel fashion.
In accordance with a latch signal (LAT) sent from the non-illustrated interface circuit, the latch circuit 101 latches the bit map data output from the shift register 100, and outputs the latched bit map data corresponding to one line to the AND gate 102.
The AND gate 102 generates a driving signal which is the logical product of the bit map data output from the latch circuit 101 and a strobe signal (STB) sent from the non-illustrated interface circuit, and outputs the driving signal to the LED array 104 through the buffer 103.
The LED array 104 makes its own LEDs emit light in accordance with the driving signal which has been output from the AND gate 103 through the buffer 103. The LED array 104 optically writes image information on its corresponding photosensitive drum by illuminating the drum with the LED light in accordance with the driving signal which has been output from the AND gate 102 through the buffer 103.
The color electrophotographic recording apparatus forms electrostatic latent images on the photosensitive drums by making the optical writing heads of the image forming units perform the optical writing. The color electrophotographic recording apparatus makes the image forming units adhere yellow (Y) toner, magenta (M) toner, cyan (C) toner and black (BK) toner to the formed electrostatic latent images, and transfers the toner images adhering to the photosensitive drums to a sheet of recording paper so that the toner images are superimposed on one another. The color electrophotographic recording apparatus fuses the transferred toner images to the sheet of recording paper, thus printing a color image.
In order to form a color image of high quality, however, the color electrophotographic recording apparatus needs to precisely form the images in the individual colors and accurately superimpose those images on one another. The image forming units are required to precisely form the electrostatic latent images on the photosensitive drums and accurately transfer the toner images or the electrostatic latent images bearing the toners to a sheet of recording paper so that those images are accurately superimposed on one another on the sheet of recording paper.
Even if the image superimposing accuracy at the time of the image transfer is improved, the color electrophotographic recording apparatus cannot form a high quality color image unless each optical writing head optically writes an electrostatic latent image in the accurate position on its corresponding photosensitive drum. When the optical writing accuracy is low, each optical writing head cannot form an electrostatic latent image in the accurate position on the corresponding photosensitive drum. This results in the formation of a low quality color image. The precision of the arrangement of the LEDs forming LED arrays and the precision of the illumination of the LED light influence the optical writing accuracy. When the precision of the arrangement of the LEDs and the precision of the illumination of the LED light are low, each optical writing head forms a low quality image deviating from the intended position.
The LEDs need to be highly precisely aligned with each other along a sub scanning direction or the direction in which recording paper is conveyed. For example, in the case of printing an image with a resolution of 300 dpi on a A3-sized sheet (a print width of approx. 300 mm) through utilization of 84.7 .mu.m.times.84.7 .mu.m LEDs, approximately 3500 LEDs need to be aligned with each other in each optical writing head.
An explanation will now be made in regard to an electrostatic latent image which an optical writing head forms when some of the LED array chips forming the LED array 104 are out of alignment and deviate from the designed positions in the sub scanning direction as shown in FIG. 24A.
In the LED array 104, LED array chips Nos. .left brkt-top.1.right brkt-bot., .left brkt-top.4.right brkt-bot. and .left brkt-top.8.right brkt-bot. are arranged along the "0" line as shown in FIG. 24A. LED array chip No. .left brkt-top.2.right brkt-bot. deviates by "1" from the "0" line in a "-" direction, LED array chip No. .left brkt-top.3.right brkt-bot. deviates by "2" from the "0" line in the "-" direction, LED array chip No. .left brkt-top.5.right brkt-bot. deviates by "1" from the "0" line in a "+" direction, LED array chip No. .left brkt-top.6.right brkt-bot. deviates by "2" from the "0" line in the "+" direction, and LED array chip No. .left brkt-top.7.right brkt-bot. deviates by "1" from the "0" line in the "+" direction.
Under the above-described conditions, the optical writing head forms a electrostatic latent image deformed as illustrated in FIG. 24C, not the intended image "A" shown in FIG. 24B. The optical writing head illuminates its corresponding photosensitive drum with the LED light which deviates in the sub scanning direction in correspondence with the positional deviations of LED array chips, and as a result, forms a deformed electrostatic latent image such as that shown in FIG. 24C on the corresponding photosensitive drum.
In the case of illuminating each photosensitive drum with the LED light coming from the corresponding LED array 104 through lenses such as a convergent photoconductor array or the like, a high order of straightforwardness of the LED light passing through the lenses is required of the optical writing heads. However, the LED light passing through the lenses skews, although the amount of skew is on the order of 0.1 mm or less and thus the skew is so small as cannot be recognized by human eyes.
The optical writing heads of the image forming units are also required to be arranged precisely in their respective designed positions. Even if the optical writing heads optically write images with high accuracy on their corresponding photosensitive drums, the images will not constitute a high quality color image on a sheet of recording paper unless the images exactly overlap each other on the sheet of recording paper. A low quality color image, made up of those images in individual colors which do not exactly overlap each other and deviate from a predetermined position, is formed when the arrangement of the LEDs deviates from the designed position in the sub scanning direction or when a .theta.-directional deviation (deviation in a direction around a vertical axis perpendicular to a direction in which the LEDs are arranged) occurs. The ".theta.-directional deviation" is the phenomenon wherein one end and the other end of the arrangement of the LEDs are different from each other in the amount of deviation in the sub scanning direction.
Such a low quality color image is formed also when the rotational speeds of the photosensitive drums differ from each other because of any manufacturing errors of a system for driving the photosensitive drums or the eccentricity of the photosensitive drums, and when a speed variation occurs in a conveyor system such as a conveyor belt, etc.
High quality optical writing heads which are free from the above-described drawbacks and wherein the arrangement of the LEDs and the illumination of the LED light are precise, are difficult to manufacture. The manufacturing of such optical writing heads takes long time, and the manufacturing yield is low accordingly.